There are a number of different considerations for manufacturing a resin suitable for use in rotomolding manufacture. The resin needs to be: capable of production at commercially acceptable rates of production; suitable for use in the rotomolding process (e.g., for example, having a suitable sintering temperature and a suitable cooling rate to be removed from the mold) and finally have suitable properties for the end use application. One important property sought is environmental stress crack resistance. The resin should not develop cracks due to exposure to chemicals, sunlight, etc. in applications, such as, tank sprayers for agricultural use, cisterns, and smaller rotomolded parts.
U.S. Pat. Nos. 5,382,630, and 5,382,631 issued Jan. 17, 1995 to Stehling, assigned to Exxon, teach bimodal resins having superior physical properties. The patent requires that the blend have two or more components each having a polydispersity (Mw/Mn) less than 3 and the blend having a polydispersity greater than 3 and no component in the blend having a relatively higher molecular weight and a lower comonomer content (i.e., the comonomer incorporation is reverse). The reference does not suggest improved ESCR.
U.S. Pat. No. 6,969,741 issued Nov. 29, 2005 to Lustiger et al., assigned to ExxonMobil teaches a blend of polyethylenes suitable for rotomolding. The patent teaches the difference in the density of each component is not less than 0.030 g/cm3. The difference in the densities of the component polymers in the present composition is less than 0.030 g/cm3.
U.S. Pat. No. 8,486,323 issued Jul. 16, 2013 in the name of Davis, assigned to Dow Global technologies Inc., teaches polymer blends used in rotational molded articles and having a high impact resistance. The blends have a residual unsaturation of less than 0.06 per 1000 carbon atoms. Embodiments in the present disclosure have a residual unsaturation of greater than 0.06 per 1000 carbon atoms.
U.S. Pat. No. 8,492,498 issued Jul. 23, 2013 from an application filed Feb. 21, 2011 in the name of Buck et al., assigned to Chevron Phillips discloses a high density polymer suitable for rotational molding that has a bent strip ESCR condition A greater than 1000 hours, as determined by ASTM D 1693 in 100% IGEPAL CO-630. The polymer also has a primary structure parameter (PSP2) of greater than 8.9. Embodiments in the present disclosure have a primary structure parameter (PSP2) of from 4 to 7.
U.S. Pat. No. 8,114,946 issued Feb. 14, 2012, and U.S. Pat. No. 8,475,899 issued Jul. 2, 2013, both claiming a priority date of Dec. 18, 2008 in the name of Yang et al., assigned to Chevron Phillips teach a polymer prepared using a bridged metallocene catalyst and having a long chain branch (LCB) content of less than 0.008 per 1000 carbon atoms, by implication LCB are present in the polymer. The catalyst and process used to make the compositions of the present disclosure do not produce detectable long chain branching.
The present disclosure seeks to provide a high density polyethylene resin having exceptional environmental stress crack resistance (ESCR) and good flow properties. Flow properties are important for rotomolding resins as the resin must soften and flow in the mold. If the flow properties are too low the resin does not sinter together in a reasonable amount of time and the product cannot be made in an economical manner.